1. Field of the Invention
The present invention relates to a motor control device, and more particularly to a motor control device for reducing the generation of the noises and surge currents when the rotation speed control signal acutely varies.
2. Description of the Related Art
FIG. 1 shows a conventional motor control device for a motor. The conventional motor control device 1 includes rotation speed control circuit 11, voltage transforming circuit 12, and driving circuit 13. The rotation speed control circuit 11 provides and transmits a rotation speed control signal to the voltage transforming circuit 12. The voltage transforming circuit 12 transforms the rotation speed control signal to a direct current (DC) voltage signal and transmits the DC voltage signal to the driving circuit 13. When receiving the DC voltage signal, the driving circuit 13 drives the motor 2 to start so as to allow the fan 3 to rotate.
As shown in FIG. 2 and FIG. 3, when the rotation speed control signal acutely varies, some unwanted problems occur. For example, when the duty cycle of the rotation speed control signal changes from 40%, such as that indicated by A in FIG. 2, to 100%, the rotation speed of motor is also changed from a low speed to a high speed, such as that shown by B in FIG. 2, it causes some issues. Firstly, the acute change of the rotation speed causes large noises. Secondly, when the rotation speed control signal acutely varies, a surge current occurs, shown in FIG. 3, which could damage the motor. FIG. 3 shows the current variation schematic diagram when the rotation speed control signal acutely varies, wherein the maximum current is 3.88 (A). The vertical axis in FIG. 3 means the amount of current which flows through the coil in the motor.